Dim lightbulb?????

Hi,everyone.
I have 1 question about lightbulb dim when i press the brake pedal,
I don’t know it’s normal.

The story below,
Engine started and park the car with N gear position,
Headlight opened(low beam or parking light),
RPM is around 1000,
Voltage reading at 13.3-13.6…
When i press the brake pedal,
RPM will drop around 100,
Voltage will drop be come 13.0-13.3,
Then up back to 13.3-13.6,
Dashboard light and head light will dim a while,
May be just 0.1 second?

Do you think,
It’s normal?
Because of DA have 4 pieces of brake light bulb?